Sr7b

(Loegering and Sears, 1966) (Plate 3-5)

Low Infection Type

2 to 3-.

Environmental Variability

Low.

Origin

Common wheat; present in the Stakman et al. (1962) differentials Marquis C.I.3641 and Kota C.I.5878.

Pathogenic Variability

Virulence occurs at high frequency in most geographic areas (Luig, 1983). Moderate to high levels of avirulence may be found in parts of southern Africa (Huerta-Espino, 1992), the USA and Australia.

Reference Stocks

i: ISr7b-Ra C.I. 14165 (Loegering and Harmon, 1969).

s: Chinese Spring*6/Hope 4A (Sears et al., 1957).

v: Marquis Sr18 Sr19 Sr20 (Knott, 1965).

Source Stocks

Sr7b is very common and is not fully catalogued in many wheat cultivars [see list in Luig (1983)].

Australia: Spica Sr17.

Canada: Red Bobs Sr10 (Dyck and Green, 1970).

CIMMYT: Kiric 66 Sr6 (McVey and Roelfs, 1975).

Europe: Fertödi 293 (Luig, 1983); Halle 9H39 (Luig, 1983). Roussalka Sr8a (McVey and Roelfs, 1975).

USA: Geneva; Hart; TAM102. Hope Sr2 Sr9d Sr17 (Sears et al., 1957). Caldwell Sr9d Sr10. Marfed Sr10. Nell Sr17 (Wells et al., 1983). Ceres Sr28; Kota Sr28 (McIntosh, 1978). C.I.12632 Sr36.

 

Seedling leaves of (L to R): C.I.12632 = W1656, Marquis, Spica, CS/Hope 4A and Chinese Spring; infected with A. pt. 98-1, 2, 3, 5, 6 [P7bp17P36], B. pt. 343-1, 2, 3, 4, 5, 6 [P7bp17p36] and C. pt. 21-1, 2, 3, 7 [p7bP17P36]. Sets A. and B. were incubated at 20°C whereas set C. was incubated at 18°C to ensure that resistance would be conferred by Sr17. The resistance of C.I.12632 (leaf 1) in A. and C. is conferred by Sr36; the resistance of Spica (leaf 3) in C. is conferred by Sr17.

 

Use in Agriculture

Sr7b has not been consciously selected as a source of stem rust resistance. Avirulence on plants with Sr7b was probably present when rust research began in Australia in the 1920s. It disappeared with the extinction of pathotypes present at that time, only to reappear in 1968 with putative introductions from southern Africa (Watson and de Sousa, 1983). The current predominant Australian pathotype is avirulent (Park and Wellings, 1992) and Sr7b confers an effective degree of resistance.